Rootcanal: Use packets for LinkLayerSocket size

Bug: 138260499
Test: connect two Rootcanal instances and discover them
      adb -s $DEVICE_1 forward tcp:6111 tcp:6111
      adb -s $DEVICE_2 forward tcp:6112 tcp:6111
      adb -s $DEVICE_1 forward tcp:6311 tcp:6311
      adb -s $DEVICE_2 reverse tcp:7000 tcp:6311

      Connect the test console to phone 2 and connect them together
      python scripts/test_channel.py 6112
      $ add_remote localhost 7000 BR_EDR
      $ quit
Change-Id: Ie3d1d83c42e9766db8f0efb65c31301acfbba170